Re: [PATCH v2 8/9] ip: Track observed guest IPv4 addresses in unified address array

On Sun, 18 Jan 2026 17:16:11 -0500 Jon Maloy wrote:

We remove the addr_seen field in struct ip4_ctx and replace it by setting a new INANY_ADDR_OBSERVED flag in the corresponding entry in the new address array. If the seen address is not present in the array we add it first.

Signed-off-by: Jon Maloy --- conf.c | 3 --- fwd.c | 49 ++++++++++++++++++++++++++++++++++++++----------- inany.h | 1 + migrate.c | 44 ++++++++++++++++++++++++++++++++++++++++++-- passt.h | 3 --- tap.c | 38 +++++++++++++++++++++++++++++++++----- 6 files changed, 114 insertions(+), 24 deletions(-)

diff --git a/conf.c b/conf.c index 22c2222..8f0091d 100644 --- a/conf.c +++ b/conf.c @@ -787,8 +787,6 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4) ip4->addrs[i].prefix_len = ip4_default_prefix_len(a4); }

- ip4->addr_seen = *inany_v4(&ip4->addrs[0].addr); - ip4->our_tap_addr = ip4->guest_gw;

if (!ip4->addr_count) @@ -804,7 +802,6 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4) static void conf_ip4_local(struct ip4_ctx *ip4) { ip4->addrs[0].addr = inany_from_v4(IP4_LL_GUEST_ADDR); - ip4->addr_seen = *inany_v4(&ip4->addrs[0].addr); ip4->our_tap_addr = ip4->guest_gw = IP4_LL_GUEST_GW; ip4->addrs[0].prefix_len = IP4_LL_PREFIX_LEN; ip4->addr_count = 1; diff --git a/fwd.c b/fwd.c index f1db34c..08b0b83 100644 --- a/fwd.c +++ b/fwd.c @@ -491,6 +491,29 @@ static bool is_dns_flow(uint8_t proto, const struct flowside *ini) ((ini->oport == 53) || (ini->oport == 853)); }

+/** + * fwd_guest_addr4() - Get first observed IPv4 guest address + * @c: Execution context + * + * Return: pointer to first observed IPv4 address, or first address if none + * observed, or NULL if no addresses

Well, it's kind of obvious it's a pointer. What about: * Return: first observed IPv4 address if any, first configured if any, or NULL ?

+ */ +static const struct in_addr *fwd_guest_addr4(const struct ctx *c) +{ + int i; + + /* Find first observed address */ + for (i = 0; i < c->ip4.addr_count; i++) + if (c->ip4.addrs[i].flags & INANY_ADDR_OBSERVED) + return inany_v4(&c->ip4.addrs[i].addr); + + /* Fallback to first address */ + if (c->ip4.addr_count > 0) + return inany_v4(&c->ip4.addrs[0].addr); + + return NULL; +} + /** * fwd_guest_accessible4() - Is IPv4 address guest-accessible * @c: Execution context @@ -515,17 +538,11 @@ static bool fwd_guest_accessible4(const struct ctx *c, if (IN4_IS_ADDR_UNSPECIFIED(addr)) return false;

- /* Check against all configured guest addresses */ + /* Check against all guest addresses */ for (i = 0; i < c->ip4.addr_count; i++) if (IN4_ARE_ADDR_EQUAL(addr, inany_v4(&c->ip4.addrs[i].addr))) return false;

- /* Also check addr_seen: it tracks the address the guest is actually - * using, which may differ from configured addresses. - */ - if (IN4_ARE_ADDR_EQUAL(addr, &c->ip4.addr_seen)) - return false; - return true; }

@@ -768,10 +785,16 @@ uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto, * match. */ if (inany_v4(&ini->eaddr)) { - if (c->host_lo_to_ns_lo) + const struct in_addr *guest_addr;

cppcheck argues that: fwd.c:1140:26: style: The scope of the variable 'guest_addr' can be reduced. [variableScope] const struct in_addr *guest_addr; ^

+ + if (c->host_lo_to_ns_lo) { tgt->eaddr = inany_loopback4; - else - tgt->eaddr = inany_from_v4(c->ip4.addr_seen); + } else { + guest_addr = fwd_guest_addr4(c); + if (!guest_addr) + return PIF_NONE; + tgt->eaddr = inany_from_v4(*guest_addr); + } tgt->oaddr = inany_any4; } else { if (c->host_lo_to_ns_lo) @@ -803,7 +826,11 @@ uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto, tgt->oport = ini->eport;

if (inany_v4(&tgt->oaddr)) { - tgt->eaddr = inany_from_v4(c->ip4.addr_seen); + const struct in_addr *guest_addr = fwd_guest_addr4(c); + + if (!guest_addr) + return PIF_NONE; + tgt->eaddr = inany_from_v4(*guest_addr); } else { if (inany_is_linklocal6(&tgt->oaddr)) tgt->eaddr.a6 = c->ip6.addr_ll_seen; diff --git a/inany.h b/inany.h index 07bfc3d..a334da9 100644 --- a/inany.h +++ b/inany.h @@ -300,6 +300,7 @@ int inany_prefix_pton(const char *src, union inany_addr *dst, int *prefix_len); /* Flags for struct inany_addr_entry */ #define INANY_ADDR_CONFIGURED (1 << 0) /* User set via -a */ #define INANY_ADDR_HOST (1 << 1) /* From host interface */ +#define INANY_ADDR_OBSERVED (1 << 2) /* Seen in guest traffic */

/** * struct inany_addr_entry - Unified IPv4/IPv6 address entry diff --git a/migrate.c b/migrate.c index 48d63a0..01be6f1 100644 --- a/migrate.c +++ b/migrate.c @@ -57,8 +57,16 @@ static int seen_addrs_source_v1(struct ctx *c, struct migrate_seen_addrs_v1 addrs = { .addr6 = c->ip6.addr_seen, .addr6_ll = c->ip6.addr_ll_seen, - .addr4 = c->ip4.addr_seen,

This requires a new version of the migration data, and a specific function to handle migration from v1 to v2. If it's trivial, migration from v2 to v1 (keeping a single address) might also be convenient.

}; + int i; + + /* Find first observed IPv4 address */ + for (i = 0; i < c->ip4.addr_count; i++) { + if (c->ip4.addrs[i].flags & INANY_ADDR_OBSERVED) { + addrs.addr4 = *inany_v4(&c->ip4.addrs[i].addr); + break; + } + }

(void)stage;

@@ -70,6 +78,33 @@ static int seen_addrs_source_v1(struct ctx *c, return 0; }

+/** + * migrate_observed_addr4() - Add observed IPv4 address to ip4_ctx address array + * @c: Execution context + * @addr: IPv4 address to add + */ +static void migrate_observed_addr4(struct ctx *c, const struct in_addr *addr) +{ + int i; + + if (IN4_IS_ADDR_UNSPECIFIED(addr)) + return; + + for (i = 0; i < c->ip4.addr_count; i++) { + if (IN4_ARE_ADDR_EQUAL(addr, inany_v4(&c->ip4.addrs[i].addr))) { + c->ip4.addrs[i].flags |= INANY_ADDR_OBSERVED; + return; + } + } + + if (c->ip4.addr_count < IP4_MAX_ADDRS) { + c->ip4.addrs[c->ip4.addr_count].addr = inany_from_v4(*addr); + c->ip4.addrs[c->ip4.addr_count].prefix_len = 0; + c->ip4.addrs[c->ip4.addr_count].flags = INANY_ADDR_OBSERVED; + c->ip4.addr_count++; + } +} + /** * seen_addrs_target_v1() - Receive and use guest observed addresses on target * @c: Execution context @@ -82,6 +117,7 @@ static int seen_addrs_target_v1(struct ctx *c, const struct migrate_stage *stage, int fd) { struct migrate_seen_addrs_v1 addrs; + struct in_addr addr4;

(void)stage;

@@ -90,7 +126,11 @@ static int seen_addrs_target_v1(struct ctx *c,

c->ip6.addr_seen = addrs.addr6; c->ip6.addr_ll_seen = addrs.addr6_ll; - c->ip4.addr_seen = addrs.addr4; + + /* Avoid alignment warning */ + addr4 = addrs.addr4; + migrate_observed_addr4(c, &addr4); + memcpy(c->guest_mac, addrs.mac, sizeof(c->guest_mac));

return 0; diff --git a/passt.h b/passt.h index 929b474..db09da5 100644 --- a/passt.h +++ b/passt.h @@ -68,7 +68,6 @@ enum passt_modes { * struct ip4_ctx - IPv4 execution context * @addrs: IPv4 addresses assigned to guest * @addr_count: Number of addresses in addrs[] array - * @addr_seen: Latest IPv4 address seen as source from tap * @guest_gw: IPv4 gateway as seen by the guest * @map_host_loopback: Outbound connections to this address are NATted to the * host's 127.0.0.1 @@ -87,7 +86,6 @@ struct ip4_ctx { struct inany_addr_entry addrs[IP4_MAX_ADDRS]; int addr_count;

- struct in_addr addr_seen; struct in_addr guest_gw; struct in_addr map_host_loopback; struct in_addr map_guest_addr; @@ -127,7 +125,6 @@ struct ip6_ctx { /* PIF_TAP addresses */ struct inany_addr_entry addrs[IP6_MAX_ADDRS]; int addr_count; -

Unrelated change.

struct in6_addr addr_seen; struct in6_addr addr_ll_seen; struct in6_addr guest_gw; diff --git a/tap.c b/tap.c index 7c50013..1a52adb 100644 --- a/tap.c +++ b/tap.c @@ -161,6 +161,37 @@ void tap_send_single(const struct ctx *c, const void *data, size_t l2len) } }

+/** + * tap_check_src_addr4() - Note an IPv4 address seen in guest traffic + * @c: Execution context + * @addr: IPv4 address seen as source from guest + * + * Add the address to ip4.addrs[] with OBSERVED flag if not already present.

Nit: we don't add . at the end of those descriptions (they're not always full sentences).

+ */ +static void tap_check_src_addr4(struct ctx *c, const struct in_addr *addr) +{ + int i; + + /* Check if already in array */ + for (i = 0; i < c->ip4.addr_count; i++) { + if (IN4_ARE_ADDR_EQUAL(addr, inany_v4(&c->ip4.addrs[i].addr))) { + c->ip4.addrs[i].flags |= INANY_ADDR_OBSERVED; + return; + } + } + + /* Add new entry if space available */ + if (c->ip4.addr_count < IP4_MAX_ADDRS) { + c->ip4.addrs[c->ip4.addr_count].addr = inany_from_v4(*addr); + c->ip4.addrs[c->ip4.addr_count].prefix_len = 0; + c->ip4.addrs[c->ip4.addr_count].flags = INANY_ADDR_OBSERVED; + debug("added new IPv4 address at index %d", c->ip6.addr_count); + c->ip4.addr_count++; + } else { + warn("IPv4 address table full, can't add new address");

This is a rather convenient vector for a denial of service: just have passt logging to syslog (default) and then a guest cycling over a set of addresses. It should be debug() instead. And I guess we actually want to reuse the least recently used (_OBSERVED) entry. Given that the array is short, we could keep it ordered, too.

+ } +} + /** * tap_ip6_daddr() - Normal IPv6 destination address for inbound packets * @c: Execution context @@ -771,8 +802,8 @@ resume: continue; }

- if (iph->saddr && c->ip4.addr_seen.s_addr != iph->saddr) - c->ip4.addr_seen.s_addr = iph->saddr; + if (iph->saddr) + tap_check_src_addr4(c, (const struct in_addr *)&iph->saddr);

if (!iov_drop_header(&data, hlen)) continue; @@ -950,9 +981,6 @@ resume: if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr_seen)) { c->ip6.addr_seen = *saddr; } - - if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addrs[0].addr.a6)) - c->ip6.addrs[0].addr.a6 = *saddr; } else if (!IN6_IS_ADDR_UNSPECIFIED(saddr)){ c->ip6.addr_seen = *saddr; }

-- Stefano